a. Field of the Invention
This invention relates to brake drums. In particular, the invention relates to a brake drum having nested inner and outer brake drums made from different materials to take advantage of different material properties.
b. Background Art
A conventional drum brake includes a brake drum that rotates with a wheel or wheels proximate to one end of an axle. The drum defines a radially inner braking surface. A brake spider is disposed about the axle and a pair of brake shoes are pivotally mounted at one end to the brake spider. The opposite end of each brake shoe is engaged by an actuating member such as a cam or hydraulic piston or wedge to move the brake shoes between positions of engagement and disengagement with the braking surface of the brake drum.
Conventional brake drums have a number of drawbacks. First, conventional brake drums often contribute to brake fade. Brake fade occurs when the brake actuator nears the furthest extent of its possible travel. In this situation, the force output of the brake actuator decreases as well as the resulting brake torque. Conventional brake drums are subject to ovalization which causes the brake actuator to travel further in order to maintain contact between the brake shoes and brake drum while also creating non-uniform contact with the brake linings. Conventional brake drums also retain excessive heat resulting in thermal expansion of the drum and requiring further travel of the actuator. Excessive temperatures also reduce the friction of brake lining materials, further contributing to brake fade.
Second, various portions of conventional brake drums heat and cool at different rates. This causes thermal stress on the drum which, along with mechanical stress, leads to cracks in the wall of the brake drum.
Third, conventional brake drums have sections that are relatively thick. When the braking friction surface of such a section is heated rapidly, its expansion is constrained by cooler portions of the section causing the warmer surface to yield in compression. When the section cools to a more uniform temperature, contraction of the formerly warm surface is again constrained by the remainder of the section causing the generation of surface cracks. The constrained expansion and contraction of the friction surface in conventional brake drums leads to shallow surface cracks or “heat checks.”
Fourth, conventional brake drums are relatively heavy. Most conventional brake drums are machined from gray iron castings. Graphite flakes in gray iron provide material damping and provide relatively stable friction and wear properties, but graphite flakes have planar discontinuities that result in low strength, brittleness and low stiffness. As a result, a relatively heavy design weight is required for gray iron brake drums. Further, conventional brake drums require a relatively large mass to act as a heat sink and control the temperature of the brake and surrounding structures. Materials including ductile iron, vermicular compacted graphite iron, steel, aluminum and aluminum matrix composites have been used in brake drums in an effort to overcome these deficiencies, but each of these materials have their own disadvantages in terms of poor friction and wear properties, low material damping and stiffness, high thermal stress and low strength.
The inventor herein has recognized a need for a brake drum that will minimize and/or eliminate one or more of the above-identified deficiencies.